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SubjectRe: [PATCH] Fix race/oops in tty layer after BKL pushdown
Date
Am Donnerstag, 7. August 2008 schrieb Alan Cox:
> > It seems that tty was already free in disassocate_ctty when it tries
> > to dereference tty->driver.
> >
> > After moving the lock_kernel before the mutex_unlock, I can no longer
> > reproduce the problem.
> >
> > Please review and consider to apply:
>
> This doesn't help as the BKL doesn't protect tty here - we don't have
> refcounting on the ttys yet so this bug (which goes back forever) simply
> becomes a different sized race if you swap the lock ordering.
>
> Given tty_vhangup just fires off a wait queue which is killed on the tty
> destruction you should be able for now at least to move the mutex_unlock
> to after the call to tty_vhangup().
>
> Does that also fix the problem ?

You mean something like the below patch? Yes, that works as well.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

---
drivers/char/tty_io.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

Index: kvm/drivers/char/tty_io.c
===================================================================
--- kvm.orig/drivers/char/tty_io.c
+++ kvm/drivers/char/tty_io.c
@@ -1161,12 +1161,11 @@ void disassociate_ctty(int on_exit)
tty = get_current_tty();
if (tty) {
tty_pgrp = get_pid(tty->pgrp);
- mutex_unlock(&tty_mutex);
lock_kernel();
- /* XXX: here we race, there is nothing protecting tty */
if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY)
tty_vhangup(tty);
unlock_kernel();
+ mutex_unlock(&tty_mutex);
} else if (on_exit) {
struct pid *old_pgrp;
spin_lock_irq(&current->sighand->siglock);

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